This invention relates generally to sensing power line current and, more particularly, to self powered current sensors for use in circuit breakers.
Many electrical and electronic systems and devices include current sensors for sensing current in a conductor. For example, power distribution systems include components such as circuit breakers, transformers and power lines. A typical circuit breaker includes current sensors for identifying transients in the power line current and controls for determining when to trip, i.e., open, a particular branch of the system. More specifically, the power line carries an input current Iin. A current sensor or comparator senses the input current and provides a scaled output current Iout having a magnitude proportional to, but many magnitudes lower than, the input current. Scaled output current Iout is used to identify transients and to determine when to trip the system.
A known current sensor, also referred to herein as a comparator, includes a current transformer having a core of magnetic material and secondary windings. Each winding has a large number of turns of fine gauge wire evenly distributed around the core. The core encircles the power line carrying input current Iin.
In operation, an alternating magnetic flux from the power line carrying current Iin is induced in the current comparator core. A voltage is therefore induced in the secondary windings of the comparator and is provided to, for example, a high gain differential amplifier. The amplifier output signal is supplied to a feedback winding to obtain a condition of zero-flux in the core. The current in the feedback winding is then the scaled output current Iout.
Accurate current sensing is often required in, for example, circuit breaker applications in which digital display and metering is employed. Inaccurate current sensing could lead, for example, to unnecessarily cutting-off power to a load and power metering discrepancies, which are undesirable. Although the above known sensor provides acceptable results, it would be desirable to improve the sensing accuracy of the sensor while reducing its cost.
In addition to high accuracy and low cost, the physical size of current sensors is often important. For example, if products must be redesigned in order to incorporate a new current sensor, the cost of adding the new current sensor to the product can be very expensive. Preferably, any new current sensor is sized so that it can be easily installed into existing units, such as circuit breakers.
Further, since many power system components preferably are not battery driven, the current sensor also is preferably self powered. This generally means that any power required by the current sensing circuit and other electronic components in the breaker unit must be provided by the power line being monitored. Use of a battery would not only add to the cost of the sensor but also the power system components would be constrained to rely on battery power to provide protection in the event of an overcurrent condition requiring the breaker to trip. By providing a self powered current sensor, these added disadvantages can be avoided and high reliability can be achieved.
It would be desirable, therefore, to provide a current sensor sized so that it can be used in existing units, such as circuit breakers. Such a current sensor must also provide high accuracy, to sense current in a power line, and preferably be self-powered.
A self powered parallel bar current sensor for generating a signal which accurately represents current flowing through a power line includes, in one embodiment, a power coil with a core of magnetic material having an opening extending therethrough, and a current sensing element. A power coil is wrapped, or wound, on the power coil core. The current sensing element includes two sensing bars and a sensing coil. The sensing bars are substantially parallel to each other with the sensing coil disposed therebetween. The sensing bars and sensing coil extend through the power coil core opening.
To monitor the current in a power line, the power line is electrically coupled to the parallel sensing bars. The magnetic field then generated by current in the parallel bars induces a voltage in the current sensing coil and a current in the power coil. The induced voltage in the current sensing coil, which is proportional to the time rate of change of the current in the power conductor, is provided to a sensing circuit which then generates an output signal representative of the power conductor current.
The induced current in the power coil is used to energize the sensing circuit components as well as any additional components. For example, the power coil may be electrically connected to a power rectifier and control circuit that are connected to supply power to other components of the unit.
The current sensor of the invention provides a signal that accurately represents current in a power conductor, and yet requires low material and assembly cost. The parallel sensing bars each contribute to generation of flux in the power coil core, thus providing ample current generation for self powering. The power coil effectively shields the sensing coil from external interferences while the field generated by the parallel bars changes by 180 degrees over a short distance, thus providing further protection from external interferences.